A calender for a papermaking machine typically comprises one or more stacks of calender rolls. Each of the calender rolls in the stack is driven by friction from the calender roll immediately below it and the lowermost roll, known as the king roll, is usually the only driven roll. During roll lifting at the termination of the calendering operation lift arms lift the calender rolls into a separating position which allows for roll removal from any position without removal of other rolls or allows the rolls to cool. The weight of the calender rolls is carried by the bearings to the frame of the calender during roll lifting. During the calendering operation the rolls are lowered into engagement with each other by the lift arm and the nip pressures between the rolls and the bearing pressures associated with the weight of a roll are controlled either by lifting the roll slightly, referred to hereafter as the relief mode, or by lowering the roll slightly, referred to hereafter as the load mode.
The use of fluid pressurized actuators, such as air bags, for applying pressure to individual rolls of a stack of calender rolls is well known in the art. It is known to place a single air bag between a stationary arm and a moveable lift arm for the purpose of relieving pressures in a calender stack. The use of a single air bag is disclosed in U.S. Pat. No. 3,270,664 issued June 22, 1964 to Veneman et al and U.S. Pat. No. 3,598,041 issued Aug. 10, 1971 to De Noyer.
Canadian Patent No. 1,032,000 issued May 30, 1978 to Bryce et al and U.S. Pat. No. 4,461,159 issued Feb. 10, 1987 to Smith et al, disclose the use of an air bag between two pivotally movable arms for the purpose of loading and relieving pressures in a calender stack. In these devices the arms are moved between loading and relief positions by a hydraulic piston cylinder arrangement and linkage connected to the arms. Once the arms are moved into one of their positions, the air bag is inflated to lock the arms in position and control the nip pressure. While such devices operate effectively, these devices are complicated in construction and operation. Due to the linkage and pivoting of the arms these devices have inherent frictional force losses which may adversely effect accurate transfer of the forces from the device to the roll nip. Also, switching the device from a pressure relief mode to a load mode and vice versa is complicated by the arms having to be pivoted by the piston/cylinder via the linkage Further, the vertical height requirement of this type of device is greater than the height of one roll due to pivoting linkage requiring more space.
An apparatus incorporating the use of two air bags operating in opposition to one another is disclosed in U.S. Pat. No. 3,158,088 issued Nov. 24, 1964 to Seidel. This Patent discloses a pivoting C-shaped lever for controlling loading and lifting of a calender lift arm. The lever has two arms to which air bags are secured. A stationary abutment is mounted to the calender frame and is sandwiched between the air bags. The air bags are selectively pressurized to effect pivotal movement of the lever thereby transferring the forces to the calender roll via the roll lift arm. This device is subject to similar problems as those noted above in respect of pivoting arms.
Another apparatus using two air bags to control calender pressures is disclosed in U.S. Pat. No. 3,525,301 issued Aug. 25, 1970 to Justus et al. This patent teaches the use of two pressurized air bags located one above the other with an end of the lifting arm located between the air bags for the purpose of loading and relieving the bearing and nip pressures of the calender stack. In practice the lower air bag must be considerably higher than the upper air bag to allow sufficient downward travel of the roll lift arm during roll separation when calendering has been terminated. This together with the fact that the height requirement of the apparatus is dictated by the cumulative heights of the two air bags, interposed lifting arm end, and mounting brackets it appears that only alternating lift arms can be operated in the load and relief modes of operation.